The engaging pieces of prior art zippers, especially lower engaging pieces, possibly shift in the manufacturing process since the zipper strips cannot be precisely located on the upper and lower engaging piece molds so that the upper and lower engaging pieces cannot be precisely positioned in the molds. Therefore, the upper and lower engaging pieces have bad appearance.
With reference to FIGS. 1 and 2, the prior art way for manufacturing a zipper is illustrated. The prior art zipper has a left and a right zipper strips 90. Each zipper strip 90 has zipper teeth 91 thereon. A part of the zipper teeth 91 is scraped with a predetermined length. Then two layers of films are melt and then coated on the two sides of the zipper strips 90. Then the two layers of films are punched with notches 93 which are opened. In the punching process, the connecting strip 901 is also punched. Then, lower engaging pieces 94, 95 are injection-molded on the notches, and the upper engaging pieces 96, 97 at the upper side of the films 92 are injected.
In above process, the two layers of films are used to fix the lower engaging pieces of the zipper 9. Other than combining the lower engaging pieces 94, 95 to the films 92, the fingers may pinch this part. The object of forming the notches 93 is that the lower engaging pieces 94, 95 will extend to the notches 93 after they are injection-molded (referring to FIG. 2) so that the bottoms of the lower engaging pieces will not enclose the zipper strips and the films 92. Therefore, the strength of the bottom is larger than other part.
Referring to FIG. 3, the shaping of the lower engaging pieces 94, 95 and the upper engaging pieces 96, 97 must guide the zipper strips into the molds. When the zipper strips are guided into molds so that the zipper strips are precisely positioned in the upper and lower molds. The zipper strips 90 must be tensioned within the mold so as to become straight and thus flat on the molds. However, the films 92 will deform as the zipper strips 90 are tensioned within the mold due to the action of the notches 93 with openings on the films 92 of the zipper strips 90. These notches will cause the widths of the films become smaller. Therefore, the zipper strips are tensioned within the mold and thus the notches 93 will deform. In general the rectangular notches will become trapezoidal notches (referring to FIG. 3) or the films 92 are bent and deformed. Therefore, the engaging pieces can not precisely match the upper and lower molds. After the lower engaging pieces 94, 95 and upper engaging pieces 96, 97 are injection-molded, the zipper strips 9 can be separated from the molds. Then the upper and lower engaging pieces of the released zipper strips 9 (referring to FIG. 4), especially the lower engaging pieces, will shift and dislocate due to the deformation of the films in the injection-molding process. As a result, it is very possible that the upper and lower engaging pieces can not be matched with one another or even a bad outlook appears.